With the development of commercially available digital computers industrial process control has been typically handled through centralized direct digital control systems. These systems included a main frame computer which is programmed to control processes. From time to time, the program would be modified to change the control of processes and to accept new process control functions or parameters. As a result, the use of computerized direct digital control of processes was expanded in order to control more of the total process than could previously been achieved with prior conventional analog process controllers. However, a number of major fundamental problems existed with such a main frame, direct digital control system. As with any complex piece of equipment, maintenance was difficult and required personnel with specialized training. As the process control became more sophisticated, it became impractical to maintain in-house service personnel for the computerized system. Therefore, users had to rely upon vendors for maintenance and repair support. Moreover, in a continuous process, the failure of the central computer could have catastrophic effects because control of a substantial portion if not all of the overall process would be interrupted. In some areas of technology, such as glass forming, reliance upon a single main frame central processing control system became impractical because of the potential of these catastrophic failures.
More recently, distributed computer control systems have been developed in which a single board computer is connected to each control device with each single board computer having a specific program dedicated to the particular control device. However, a number of drawbacks have been associated with the distributed computer control systems. In order to conserve memory and reduce operational time, the single board computers are generally programmed in a low-level, assembly-type language. As a result, many system users are reluctant to employ distributed single board computers because of the difficulty of obtaining or training personnel to program the computers. Moreover, because maintenance personnel are typically not trained in programming, a large inventory of single board computers are necessary in order to properly repair a system malfunction by replacing the microprocessor which is down. In addition, should there be a desire to upgrade or modify the program in any given microprocessor, such modification or upgrading will often require the modification of the overall system control program. Thus, cost effective retrofitting of existing industrial process control systems is not feasible.
Johnson et al. disclosed in U.S. Pat. No. 4,253,148 a system of distributed control of a process which was designed to overcome the difficulty of having a large inventory of preprogrammed microprocessors for each device to be controlled. Thus, in the Johnson et al. system a master or composite program was developed which was capable of controlling all possible logic functions and command sequences for all of the devices to be controlled. This program was downloaded into each of a plurality of microprocessors. Because each microprocessor was designed to control a specific device, a user alterable interconnection device was provided in each microprocessor in the form of a programmable read only memory (PROM). The PROM in each microprocessor allowed only those portions of the master program which related to the logic control functions and control sequences of a particular control device to be connected to the input/output ports associated with the device. Thus, as each microprocessor continuously looped through the master program, only those program steps which related to the control functions and sequences for a control device would be operative. This system had the drawback that a sometimes massive program had to be downloaded into each microprocessor. In addition, the time constraints on such a system which must loop through the entire master program is such that some processes cannot be adequately controlled. Moreover, because the master control program is stored in each microprocessor, storage space for other data such as process control data and operational parameters is limited.
It, therefore, is an object of the present invention to provide an improved distributive control, microprocessor based process control system, with interchangeable components whose programming is transparent to the operators.